1. Field of Invention
The present invention relates to power conversion, and more particularly, to a soft start system and method for a power converter.
2. Description of Related Art
Power converters are essential for many modern electronic devices. Among other capabilities, a power converter can adjust voltage level downward (buck converter and its derivatives) or adjust voltage level upward (boost converter and its derivatives). A power converter may also convert from alternating current (AC) power to direct current (DC) power, or vice versa. A power converter may also function to provide an output at a regulated level (e.g., 5.0V). Power converters are typically implemented using one or more switching devices, such as transistors, which are turned on and off to deliver power to the output of the converter. Control circuitry is provided to regulate the turning on and off of the switching devices, and thus, these converters are known as “switching regulators” or “switching converters.” Such a power converter may be incorporated into or used to implement a power supply—i.e., a switching mode power supply (SMPS). The power converters may also include one or more capacitors or inductors for alternately storing and outputting energy
Some power converters may employ a soft-start circuit in order to begin operation after power on. One kind of soft-start circuit can be a closed-loop soft-start which maintains an error amplifier of the power converter in its linear operating mode to actively control the output voltage of the power converter to follow a reference voltage VREF at the non-inverting input of the error amplifier. The power converter is able to follow the reference voltage VREF until the error amplifier output is saturated—i.e. its output is not limited by the supply rails of the error amplifier.
Typically, during start up, the reference voltage VREF at the non-inverting input of the error amplifier rises with a predetermined speed, dVREF/dt. That speed is calculated during initial circuit design and based on the following factors: (1) the value of the soft start capacitor (CSS) coupled to the non-inverting input of the error amplifier; and (2) the value of a soft start resistor (RSS) or the amplitude of an ISS dc current source coupled to the soft start capacitor for charging the same.
The rise time of the reference voltage dVREF/dt at the non-inverting input of the error amplifier determines how quickly the output capacitor COUT is charged from an initial condition of 0V to its final value, where the output voltage VOUT can be regulated by the power converter. The rise time of the output voltage dVOUT/dt is proportional to the rise time of the reference voltage dVREF/dt.
To support this rate of rise dVOUT/dt at the output during soft start, the power converter must deliver the sum of two current components. The first current component charges the output capacitor and is a function of the value of the output capacitor and rate of rise at the output (COUT*dVOUT/dt). The second current component provides power to the load (ILOAD). The total output current (IOUT) delivered by the power converter then is:
      I    OUT    =                    C        OUT            ·                        ⅆ                      V            OUT                                    ⅆ          t                      +          I      LOAD      
The equation above shows that during soft start, the output current IOUT depends not only on the dVREF/dt value calculated by the designer but also the converter's output capacitance COUT and the actual load during soft start. Furthermore, the output capacitance COUT might have a significant tolerance and its value can be easily multiplied by additional capacitance added by the end user of the power supply. In addition, a power supply is typically required to reliably start up with any load. All these effects will greatly influence the required output current IOUT during start up.
A problem arises if the above-calculated output current IOUT exceeds the maximum output current of the power converter. When this happens, the power converter can no longer operate in a closed-loop operating mode. In particular, the maximum output current is usually established by a current limit circuit and is slightly higher than the specified maximum load current ILOAD. When the current of the power converter exceeds the maximum output current limit, the error amplifier goes into saturation and looses control over the output voltage VOUT, such that the output voltage VOUT does not follow the reference voltage VREF as it should. Ultimately, when the output voltage VOUT reaches its final value, the error amplifier will need to recover from its saturated state. During this recovery, the output voltage VOUT overshoots, which is an undesired phenomenon in power supplies.